1. Field of the Invention
The present invention relates to a magnetic disk drive device for driving and rotating a magnetic disk set to a hub.
2. Related Art
A magnetic disk drive device constructed as shown in FIG. 5 has been known.
In the figure, a fixed shaft 2 is erected on a frame 1. A laminated iron core 3 is secured to a large diameter portion 2a of a lower portion of the fixed shaft 2. A coil 4 is put on the iron core 3. A pair of bearings 5 and 6 are secured to the outer surface of a small diameter portion 2b or an upper portion of the fixed shaft 2 in a state that the bearings 5 and 6 are arrayed in the axial direction of the magnetic disk drive device. The pair of bearings 5 and 6 allows the hub 7 to turn about them.
The inner rings 5a and 6a of the pair of bearings 5 and 6 are bonded to the outer surface of the fixed shaft 2. A hub 7 shaped like a hollow shaped tube is secured to the outer rings 5b and 6b of the pair of bearings 5 and 6. A magnetic disc, not shown, is put around the hub 7.
A skirt portion 7a is formed in the lower portion of the hub 7. A drive magnet 8, shaped like a ring, is fastened to the inner wall of the skirt 7a in a state that the drive magnet 8 faces the outer surface of the iron core 3 with a gap therebetween and are arranged in a circle form.
An electric wire (lead wire) 9 for applying a drive voltage to the coil 4 of the iron core 3 is led out of the motor through a through hole formed in the frame 1.
A collar 11 made of magnetic material, shaped like a hollow tube, is fit to the upper end portion of the fixed shaft 2 (as viewed in the drawing). A magnetic fluid seal 12 is applied to the opening defined between the outer surface of the collar 11 and the inner surface of the hub 7.
A magnetic fluid 12a within the inner surface of the magnetic fluid seal 12 comes in close contact with the outer surface of the collar 11. The magnetic fluid isolates the pair of bearings 5 and 6 from the outside of the magnetic disk drive device. A seal cover 13, disposed above the magnetic fluid seal 12, prevents the magnetic fluid 12a from being scattered outside.
The iron core 3 and the coil 4 put on the iron core 3 will be described with reference to FIG. 6. As shown, the iron core 3 consists of a ring-like portion 15 and a plural number of salient poles 14 radially protruded from the outer circumferential edge of the ring-like portion 15.
Each salient poles 14 consists of an arcuately extended part 16 extending in arc-shaped at an outer end portion and an arm 17 arranged between the ring-like portion 15 and the arcuately extended part 16 in such a manner that the arm 17 is narrower in width than the arcuately extended part 16. The coil 4 is wound around the arm 17 of the salient poles 14 with a predetermined number of turns.
The iron core 3 consists usually of laminated thin magnetic plates, as shown in FIG. 5. In order to prevent the motor from failing to function (viz., rendering the resultant motor inoperative), which will occur if any coil 4 is shortcircuited through the iron core 3, the surface (particularly of the arm 17 wound by the coil 4) of each of the salient poles 14 is insulated from the coil 4.
The insulating film is formed on the surface of the coil 4. However, the rubbing of the coil 4 and temperature changes when the motor is driven and stopped may crack the insulating film, resulting in poor insulation.
To avoid the latter problem, the insulating film is formed on the surface of each salient pole 14.
One of the possible ways to insulate the surface of each salient pole 14 from the coil 4 is to form an insulating film on the surface of the iron core 3 including the salient poles 14.
To realize this, an electrostatic powder coating method may be used. In this method, insulating plastic powder (e.g., epoxy resin powder) is electrostatically attached to the surface of the iron core 3. Then, it is high frequency heated to be sintered.
The insulating film thus formed is about 0.3 mm thick. However, the thickness of the insulating film hinders the reduction of the motor size.
This insulating film is nonuniform in thickness, thus producing an uneven surface.
As see from FIG. 6, the number of turns of the coil 4 is limited by the space between the proximal portions of the arms 17 of the adjacent salient poles 14. In addition to this, the large film thickness and the uneven surface of the insulating film formed by the electrostatic powder coating method make it difficult to increase the number of turns of the coil 4. Because of this, the torque of the magnetic disk drive device cannot be increased.
In the multi-pole structure having an increased number of salient poles 14, the space between the proximals of the adjacent salient poles 14 is necessarily narrowed. The coil winding work is inefficient since the salient pole having the coil already put thereon is an obstacle to the coil winding work of the poles adjacent thereto.